A sheet-through image reading device has been used for reading an image on a sheet in various imaging machines such as copy machine and scanner, which is disclosed in JP 11-59955 (A), for example. The sheet-through image reading device has an elongated transparent platen extending transversely of the sheet and a sheet transporting mechanism for transporting the sheet through a reading position on the top surface of the platen. An image-capturing device such as CCD is provided behind and adjacent to the back surface of the platen. The sheet transporting mechanism has a feed-in roller unit mounted on an upstream side of the reading position in order to feed the sheet into the reading position and a feed-out unit mounted on a downstream side of the reading position in order to feed it out of the reading station. In operation, the sheet is fed by the feed-in roller unit into the reading position where the image supported on the sheet is read by the image-capturing device. Then, the sheet is held by the feed-out roller unit so that it is pulled out of the reading position.
As described above, according to the sheet-through image reading device, since the image capturing device is fixedly mounted in the imaging apparatus, a change in the sheet transporting speed will damage a quality of a resultant image. In particular, the speed change in a full color image reading device with three CCDs corresponding to three color images (e.g., Red, Green and Blue) and positioned at different positions results in a color displacement, i.e., inaccurate superposition of three different color images, which considerably deteriorates the quality of resultant full color image.
In view of the forgoing, according to the image reading device with a feed-in roller unit made of paired rigid rollers, immediately after a tailing end of the sheet is released from a contact region of the rollers, the sheet running through the reading position is accelerated so quickly. This acceleration results in an unwanted speed variation of the sheet, which in turn considerably damages the image reading and therefore degrades a quality of the resultant image.
An improved feed-in roller unit capable of overcoming such problem is provided in JP 9-226,976 (A), which includes a rigid roller made of rigid material and a flexible roller made of flexible material. In particular, the feed-in roller unit is improved in that a periphery of the flexible roller is made of rubber having a hardness of 5-40 degrees. This results in that a wide nip is defined at the contact region of the rigid and flexible rollers, preventing the acceleration of the sheet.
However, it has been found that the feed-in roller unit with rigid and flexible rollers has another drawbacks that a feed speed of the sheet varies considerably while it is nipped between the rigid and flexible rollers.